The present invention relates to a catalyst for selectively oxidizing carbon monoxide present in a fuel gas fed to an anode of a solid polymer electrolyte-type fuel cell, a method for selectively removing carbon monoxide with such a catalyst, and a solid polymer electrolyte-type fuel cell system with such a catalyst.
Because a solid polymer electrolyte-type fuel cell is high in output power density, operable at low temperatures, emitting substantially no exhaust gas containing harmful substances, it is attracting much attention as a low-pollution power source not only for stationary applications but also for vehicle applications.
Generally used as a fuel gas for a fuel cell is a compressed hydrogen gas, a high-purity hydrogen gas fed from a liquid hydrogen tank, or a hydrogen-rich gas obtained by reforming various fuels (e.g. alcohols or hydrocarbons) by a reformer. When a hydrogen-rich gas is used in a low-temperature operation, for instance, at the start of an engine, however, it is known that impurities such as carbon monoxide, carbonized gas and the like present in the hydrogen-rich gas deactivate Pt contained in an anode catalyst, resulting in higher polarization and thus lower output.
In order to prevent such disadvantages, anodes are known which are obtained by alloying Pt with a noble metal such as Pd, Rh, Ir, Ru, Os, Au or the like, or with a base metal such as Sn, W, Cr, Mn, Fe, Co, Ni, Cu or the like. However, these anodes have a limitation in resistance to deactivation with CO, and are strikingly deactivated by a hydrogen-rich gas containing 100 ppm or more of carbon monoxide.
Hence, there were proposed catalysts capable of selectively removing only carbon monoxide from a hydrogen-rich gas without sacrificing hydrogen. As such catalysts for selectively oxidizing carbon monoxide, there are known catalysts obtained by supporting noble metals such as Pt or Ru on carriers such as alumina, titania, zirconia or the like. That is, oxygen is added to a hydrogen-rich gas by moles substantially equal to carbon monoxide contained in the hydrogen-rich gas, and the resultant mixed gas is contacted with the above catalyst to selectively remove carbon monoxide (U.S. Pat. No. 5,248,566, Japanese Patent Publication No. 39-21742, Japanese Patent Laid-Open No. 8-295503, and Japanese Patent Laid-Open No. 10-101302).
Reformed gases obtained by reforming oxygen-containing hydrocarbons such as methanol or the like contain several percentages of impurities such as unreacted alcohols and intermediate products of alcohol decomposition, and these impurities (oxygen-containing hydrocarbons) significantly deactivate the catalyst used. Therefore, if a high removal ratio of carbon monoxide with conventional catalysts were sought, large volumes of catalysts would be required. Fuel cells for automobile need to be as small as possible because they are accommodated in a limited space of automobile; and the catalysts used in such fuel cells are desired to show a high removal ratio of carbon monoxide for a hydrogen-rich gas even when the hydrogen-rich gas contains oxygen-containing hydrocarbons.
The present invention has been completed to solve the above-mentioned problems of the prior art, aiming at providing a catalyst for oxidizing carbon monoxide in a hydrogen-rich gas at a high selectivity even when the hydrogen-rich gas contains oxygen-containing hydrocarbons; a method for removing carbon monoxide with such a catalyst; and a solid polymer electrolyte-type fuel cell system with such a catalyst.
As a result of intensive research to achieve the above aims, the inventors have found that with an alumina hydrate carrier supporting platinum, there can be obtained a catalyst for oxidizing carbon monoxide at a high selectivity in a hydrogen-containing gas. The present invention has been completed based on the above finding.
The present invention provides a catalyst having a high selective oxidation activity for carbon monoxide in a hydrogen-containing gas, which comprises an alumina hydrate carrier and at least platinum supported thereon.
The present invention also provides a structure comprising an integral support member coated with a catalyst for selectively oxidizing carbon monoxide, the catalyst comprising an alumina hydrate carrier and at least platinum supported thereon.
The present invention further provides a method for removing carbon monoxide from a hydrogen-rich gas containing carbon monoxide, which comprises the steps of adding oxygen to the hydrogen-rich gas containing carbon monoxide in an amount necessary to oxidize at least part of carbon monoxide, and contacting the resulting mixture with the above catalyst for selectively oxidizing carbon monoxide.
The present invention further provides a solid polymer electrolyte-type fuel cell system comprising at least a reactor containing a catalyst for selectively oxidizing carbon monoxide or a structure having the catalyst coated on an integral support member, the catalyst comprising an alumina hydrate carrier and at least platinum supported thereon. In a preferred embodiment, the solid polymer electrolyte-type fuel cell system comprises a fuel container, a reformer, a shifting reactor, a reactor for selectively oxidizing carbon monoxide with the above catalyst, and a solid polymer electrolyte-type fuel cell arranged in this order.